powerpc: add little endian flag to syscall_get_arch()

Since both ppc and ppc64 have LE variants which are now reported by uname, add
that flag (__AUDIT_ARCH_LE) to syscall_get_arch() and add AUDIT_ARCH_PPC64LE
variant.

Without this,  perf trace and auditctl fail.

Mainline kernel reports ppc64le (per a058801) but there is no matching
AUDIT_ARCH_PPC64LE.

Since 32-bit PPC LE is not supported by audit, don't advertise it in
AUDIT_ARCH_PPC* variants.

See:
	https://www.redhat.com/archives/linux-audit/2014-August/msg00082.html
	https://www.redhat.com/archives/linux-audit/2014-December/msg00004.html

Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
Acked-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>